Gustatory stimuli reliably elicit ingestion or rejection behaviors. Experiments with decerebrate animals have shown that the caudal brain stem contains neural mechanisms sufficient for mediating these discriminative responses to gustatory stimuli. Anatomical experiments have delineated several regions in the brain stem reticular formation (rf) that may contain neurons involved in generating these behavioral sequences. Using neurophysiological and neuroanatomical techniques, neurons in the lateral parvicellular tegmentum of the pons and medial, magnocellular tegmental field of the caudal medulla will be investigated to determine their sensitivity to gustatory stimuli and their synaptic relationships to brain stem motor nuclei that form the final common path for ingestion and rejection responses. Neurophysiological experiments will identify neurons that can be antidromically driven from the motor nuclei. These neurons will then be tested for responsiveness to gustatory and intra-oral somatosensory stimuli. Neuroanatomical experiments using retrograde markers will determine whether these regions of the rf receive projections from known gustatory relays in the brain stem or whether further synapses are involved. Stimulation experiments will then test whether regions of the rf mapped in these first experiments can produce oral-facial components of the ingestion and rejection responses. These experiments are intended to define the neural substrates through which gustatory afferent information elicits either ingestion or rejection of substance in the oral cavity and will contribute to an understanding or rejection of substance in the oral cavity and will contribute to an understanding of how sensory information is used to regulate biologically important behaviors.